1. Field of the Art
The present invention relates to polypropylene resin expanded particles or beads from which can be prepared an expanded molded product having a uniform foam diameter, good moldability and surface appearance and excellent physical properties, particularly energy absorption property. The expanded particles of the present invention are useful for producing expanded molded products which are used for packaging containers, toys, automobile parts, core materials for helmets, shock-absorbing packaging materials, and numerous other products.
An expanded molded product obtained from polypropylene resin expanded particles has hitherto been used advantageously as a core material for bumpers, a variety of packaging materials, and the like because they have excellent chemical resistance, impact resistance, compression strain restoring ability, and other properties.
As the polypropylene resin, a propylene-.alpha.-olefin random copolymer in which propylene is copolymerized with an .alpha.-olefin such as ethylene or butene-1 is primarily used from the standpoint of its expanding property. The copolymer comprises a product obtained by polymerization with a so-called Ziegler-Natta catalyst which is obtained from a titanium chloride and an alkylaluminum.
There has also been proposed an application of a polypropylene having a syndiotactic structure obtained by using a metallocene catalyst as the base of an expanded product (Japanese Patent Laid-Open Publication No. 224832/1992). This proposal made possible the preparation of the expanded products of a propylene homopolymer which have not been obtainable from a propylene homopolymer produced by the use of a Ziegler-Natta catalyst referred to above, but the expanded product has an insufficient energy absorption property per unit weight in compression.
Those expanded molded products obtained from the conventional polypropylene resin expanded particles have not been always satisfactory, particularly, in their energy absorption property per unit weight in compression.
On the other hand, in order to improve the energy absorption property, it is possible to use a method of increasing the stiffness of a resin constituting a foam wall by controlling the comonomer content in a copolymer or by using a resin such as a homopolymer as a base resin.
However, this method, while obviously improving the energy absorption property of the expanded molded product, has been known in the art to have a problem in that, when propylene homopolymers produced by the use of what is called Ziegler-Natta catalyst are used, the expanded particles have irregular foam diameters or an extremely small foam diameter thus resulting in poor fusing between particles as the in-mold expanded molded product or a keloidal surface appearance. The method also has a disadvantage in that, when propylene copolymers with a reduced amount of a copolymer are used, an increased steam pressure is required for satisfactorily fusing the expanded particles.
Thus, the in-mold expanded molded product obtained from conventional polypropylene resin expanded particles did not satisfy simultaneously both physical properties such as energy absorption property and moldability and surface appearance.
An object of the present invention is to solve the above described problems accompanying the conventional polypropylene resin expanded particles and to provide polypropylene resin expanded particles from which in-mold expanded molded products having uniform foam diameter, good moldability and surface appearance and particularly excellent energy absorption property can be prepared.